From: "Zachary P. Landau" <kapheine@...>
Date: 2004-08-29T07:28:59+09:00
Subject: Blocking Problem

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Hello,

I'm having trouble with IO blocking when reading from a device.  It may
be hard for people to help since most people won't be able to test this
themselves, but I'll give it a shot.

I'm trying to read from the CueCat Linux driver, which sits in
/dev/scanner/cuecat.  It is sort of like a pipe, you can use File.read
and it outputs a line with a barcode (and some other information).

The problem is, I can't avoid blocking.  There is a GTK application, and
then a separate thread that tries to read barcodes and whenever it finds
one, it tells the GTK app.

So my code looks like this:

Thread.new do
    File.open(CUECAT_DEV, File::RDONLY | File::NONBLOCK) do |cuecat|
	loop do
	    id, barcode_type, barcode = cuecat.gets.split(/,/)
	    # do stuff with the barcode
	end
    end
end

When I use a named pipe instead of CUECAT_DEV, it doesn't block.  But
when I use the actual /dev/scanner/cuecat, it blocks.  Unfortunately,
blocking locks up the entire application.

I tried getting around the issue by using Gtk.timeout_add instead, but
that didn't help.

The mailing list has some posts about blocking problems, but they are
related to sockets and pipes.  As I mentioned earlier, the above code
works when CUECAT_DEV is a pipe.

--
Zachary P. Landau <kapheine@hypa.net>
GPG: gpg --recv-key 0x24E5AD99 | http://kapheine.hypa.net/kapheine.asc

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